1. Field of the Invention
The present invention relates to extensometers for simultaneously measuring axial and torsional strain in the same specimen.
2. Description of the Prior Art
Apparatus for measuring torsional and axial strain in the same specimen has previously been advanced.
Harry R. Meline, U.S. patent application Ser. No. 474,700, filed on Mar. 11, 1983 for an Axial-Torsional Extensometer, assigned to the same assignee as this application discloses an extensometer which has a coupling frame that is used for mounting two extensometers on opposite sides of a specimen, but which has softness in more degrees of freedom than the present device. If there are unnecessary degrees of freedom, under certain loading conditions oscillations can occur that will affect the signal by inducing dynamic errors in the output of the instrument. The mounting frame in the present application is different from the prior device and a different arrangement is utilized for measuring torsional strain.
A type of a "bi-axial" extensometer is shown in U.S. Pat. No. 4,251,918. It essentially measures strain along the longitudinal or loading axis of the specimen, or strains which cause changes in diameter of the specimen, but not axial and torsional strains of the specimen.
U.S. Pat. No. 4,160,325 illustrates an extensometer that is designed for simultaneously measuring axial strain and torsional deflection of a specimen subject to axial and torsional load. In this particular device, separate gage members are clamped to the specimens and cantilever type motion sensors acting between the gage members indicate relative torsional movement, or relative axial movement. The calibration of this device appears to be rather complex, and it would appear that a tendency of one of the gage members to creep relative to the other would limit the accuracy. Another type of extensometer which has flexure beams that bend during use is shown in U.S. Pat. No. 2,663,085. However, the extensometer shown in U.S. Pat. No. '085 is capable of measuring axial strains only and does not relate to the measurement of torsional strains.
U.S. Pat. No. 3,789,508 also shows an axial extensometer having cross flexure members for holding the arms in an assembly in a manner useful in measuring strains.
U.S. Pat. No. 3,402,472 shows an extensometer which comprises elongated parallel rods on opposite sides of the specimen, which have abutments that engage the specimen. The rods are supported on a cross member that spans the specimen, but the cross member is very thin in the longitudinal axial direction of the specimen, so that it may permit unwanted movements of the extensometer rods and assembly. It is intended to be flexible in that a type of flexure member is needed to preserve the parallel relationship of the long rods or legs on each side of the specimen.